Info file bison.info, produced by Makeinfo, -*- Text -*- from input
file bison.texinfo.

This file documents the Bison parser generator.

Copyright (C) 1988, 1989, 1990 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the sections entitled "GNU General Public License" and
"Conditions for Using Bison" are included exactly as in the original,
and provided that the entire resulting derived work is distributed
under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the sections entitled "GNU General Public
License", "Conditions for Using Bison" and this permission notice may
be included in translations approved by the Free Software Foundation
instead of in the original English.



File: bison.info,  Node: Interface,  Next: Algorithm,  Prev: Grammar File,  Up: Top

Parser C-Language Interface
***************************

The Bison parser is actually a C function named `yyparse'.  Here we
describe the interface conventions of `yyparse' and the other
functions that it needs to use.

Keep in mind that the parser uses many C identifiers starting with
`yy' and `YY' for internal purposes.  If you use such an identifier
(aside from those in this manual) in an action or in additional C
code in the grammar file, you are likely to run into trouble.

* Menu:

* Parser Function:: How to call `yyparse' and what it returns.
* Lexical::         You must supply a function `yylex' which reads tokens.
* Error Reporting:: You must supply a function `yyerror'.
* Action Features:: Special features for use in actions.

 

File: bison.info,  Node: Parser Function,  Next: Lexical,  Prev: Interface,  Up: Interface

The Parser Function `yyparse'
=============================

You call the function `yyparse' to cause parsing to occur.  This
function reads tokens, executes actions, and ultimately returns when
it encounters end-of-input or an unrecoverable syntax error.  You can
also write an action which directs `yyparse' to return immediately
without reading further.

The value returned by `yyparse' is 0 if parsing was successful
(return is due to end-of-input).

The value is 1 if parsing failed (return is due to a syntax error).

In an action, you can cause immediate return from `yyparse' by using
these macros:

`YYACCEPT'
     Return immediately with value 0 (to report success).

`YYABORT'
     Return immediately with value 1 (to report failure).



File: bison.info,  Node: Lexical,  Next: Error Reporting,  Prev: Parser Function,  Up: Interface

The Lexical Analyzer Function `yylex'
=====================================

The "lexical analyzer" function, `yylex', recognizes tokens from the
input stream and returns them to the parser.  Bison does not create
this function automatically; you must write it so that `yyparse' can
call it.  The function is sometimes referred to as a lexical scanner.

In simple programs, `yylex' is often defined at the end of the Bison
grammar file.  If `yylex' is defined in a separate source file, you
need to arrange for the token-type macro definitions to be available
there.  To do this, use the `-d' option when you run Bison, so that
it will write these macro definitions into a separate header file
`NAME.tab.h' which you can include in the other source files that
need it.  *Note Invocation::.

* Menu:

* Calling Convention::   How `yyparse' calls `yylex'.
* Token Values::         How `yylex' must return the semantic value
                           of the token it has read.
* Token Positions::      How `yylex' must return the text position
                           (line number, etc.) of the token, if the
                           actions want that.
* Pure Calling::         How the calling convention differs
                           in a pure parser (*note Pure Decl::.).

 

File: bison.info,  Node: Calling Convention,  Next: Token Values,  Prev: Lexical,  Up: Lexical

Calling Convention for `yylex'
------------------------------

The value that `yylex' returns must be the numeric code for the type
of token it has just found, or 0 for end-of-input.

When a token is referred to in the grammar rules by a name, that name
in the parser file becomes a C macro whose definition is the proper
numeric code for that token type.  So `yylex' can use the name to
indicate that type.  *Note Symbols::.

When a token is referred to in the grammar rules by a character
literal, the numeric code for that character is also the code for the
token type.  So `yylex' can simply return that character code.  The
null character must not be used this way, because its code is zero
and that is what signifies end-of-input.

Here is an example showing these things:

     yylex()
     {
       ...
       if (c == EOF)     /* Detect end of file. */
         return 0;
       ...
       if (c == '+' || c == '-')
         return c;      /* Assume token type for `+' is '+'. */
       ...
       return INT;      /* Return the type of the token. */
       ...
     }

This interface has been designed so that the output from the `lex'
utility can be used without change as the definition of `yylex'.



File: bison.info,  Node: Token Values,  Next: Token Positions,  Prev: Calling Convention,  Up: Lexical

Semantic Values of Tokens
-------------------------

In an ordinary (nonreentrant) parser, the semantic value of the token
must be stored into the global variable `yylval'.  When you are using
just one data type for semantic values, `yylval' has that type. 
Thus, if the type is `int' (the default), you might write this in
`yylex':

       ...
       yylval = value;  /* Put value onto Bison stack. */
       return INT;      /* Return the type of the token. */
       ...

 When you are using multiple data types, `yylval''s type is a union
made from the `%union' declaration (*note Union Decl::.).  So when
you store a token's value, you must use the proper member of the union.
If the `%union' declaration looks like this:

     %union {
       int intval;
       double val;
       symrec *tptr;
     }

then the code in `yylex' might look like this:

       ...
       yylval.intval = value; /* Put value onto Bison stack. */
       return INT;          /* Return the type of the token. */
       ...

 

File: bison.info,  Node: Token Positions,  Next: Pure Calling,  Prev: Token Values,  Up: Lexical

Textual Positions of Tokens
---------------------------

If you are using the `@N'-feature (*note Action Features::.) in
actions to keep track of the textual locations of tokens and
groupings, then you must provide this information in `yylex'.  The
function `yyparse' expects to find the textual location of a token
just parsed in the global variable `yylloc'.  So `yylex' must store
the proper data in that variable.  The value of `yylloc' is a
structure and you need only initialize the members that are going to
be used by the actions.  The four members are called `first_line',
`first_column', `last_line' and `last_column'.  Note that the use of
this feature makes the parser noticeably slower.

The data type of `yylloc' has the name `YYLTYPE'.



File: bison.info,  Node: Pure Calling,  Prev: Token Positions,  Up: Lexical

Calling for Pure Parsers
------------------------

When you use the Bison declaration `%pure_parser' to request a pure,
reentrant parser, the global communication variables `yylval' and
`yylloc' cannot be used.  (*Note Pure Decl::.)  In such parsers the
two global variables are replaced by pointers passed as arguments to
`yylex'.  You must declare them as shown here, and pass the
information back by storing it through those pointers.

     yylex (lvalp, llocp)
          YYSTYPE *lvalp;
          YYLTYPE *llocp;
     {
       ...
       *lvalp = value;  /* Put value onto Bison stack.  */
       return INT;      /* Return the type of the token.  */
       ...
     }

If the grammar file does not use the `@' constructs to refer to
textual positions, then the type `YYLTYPE' will not be defined.  In
this case, omit the second argument; `yylex' will be called with only
one argument.



File: bison.info,  Node: Error Reporting,  Next: Action Features,  Prev: Lexical,  Up: Interface

The Error Reporting Function `yyerror'
======================================

The Bison parser detects a "parse error" or "syntax error" whenever
it reads a token which cannot satisfy any syntax rule.  A action in
the grammar can also explicitly proclaim an error, using the macro
`YYERROR' (*note Action Features::.).

The Bison parser expects to report the error by calling an error
reporting function named `yyerror', which you must supply.  It is
called by `yyparse' whenever a syntax error is found, and it receives
one argument.  For a parse error, the string is always `"parse error"'.

The parser can detect one other kind of error: stack overflow.  This
happens when the input contains constructions that are very deeply
nested.  It isn't likely you will encounter this, since the Bison
parser extends its stack automatically up to a very large limit.  But
if overflow happens, `yyparse' calls `yyerror' in the usual fashion,
except that the argument string is `"parser stack overflow"'.

The following definition suffices in simple programs:

     yyerror (s)
          char *s;
     {

       fprintf (stderr, "%s\n", s);
     }

After `yyerror' returns to `yyparse', the latter will attempt error
recovery if you have written suitable error recovery grammar rules
(*note Error Recovery::.).  If recovery is impossible, `yyparse' will
immediately return 1.

The variable `yynerrs' contains the number of syntax errors
encountered so far.  Normally this variable is global; but if you
request a pure parser (*note Pure Decl::.) then it is a local
variable which only the actions can access.



File: bison.info,  Node: Action Features,  Prev: Error Reporting,  Up: Interface

Special Features for Use in Actions
===================================

Here is a table of Bison constructs, variables and macros that are
useful in actions.

`$$'
     Acts like a variable that contains the semantic value for the
     grouping made by the current rule.  *Note Actions::.

`$N'
     Acts like a variable that contains the semantic value for the
     Nth component of the current rule.  *Note Actions::.

`$<TYPEALT>$'
     Like `$$' but specifies alternative TYPEALT in the union
     specified by the `%union' declaration.  *Note Action Types::.

`$<TYPEALT>N'
     Like `$N' but specifies alternative TYPEALT in the union
     specified by the `%union' declaration.  *Note Action Types::.

`YYABORT;'
     Return immediately from `yyparse', indicating failure.  *Note
     Parser Function::.

`YYACCEPT;'
     Return immediately from `yyparse', indicating success.  *Note
     Parser Function::.

`YYBACKUP (TOKEN, VALUE);'
     Unshift a token.  This macro is allowed only for rules that
     reduce a single value, and only when there is no look-ahead token.
     It installs a look-ahead token with token type TOKEN and
     semantic value VALUE; then it discards the value that was going
     to be reduced by this rule.

     If the macro is used when it is not valid, such as when there is
     a look-ahead token already, then it reports a syntax error with
     a message `cannot back up' and performs ordinary error recovery.

     In either case, the rest of the action is not executed.

`YYEMPTY'
     Value stored in `yychar' when there is no look-ahead token.

`YYERROR;'
     Cause an immediate syntax error.  This statement initiates error
     recovery just as if the parser itself had detected an error;
     however, it does not call `yyerror', and does not print any
     message.  If you want to print an error message, call `yyerror'
     explicitly before the `YYERROR;' statement.  *Note Error
     Recovery::.

`YYRECOVERING'
     This macro stands for an expression that has the value 1 when
     the parser is recovering from a syntax error, and 0 the rest of
     the time.  *Note Error Recovery::.

`yychar'
     Variable containing the current look-ahead token.  (In a pure
     parser, this is actually a local variable within `yyparse'.) 
     When there is no look-ahead token, the value `YYEMPTY' is stored
     in the variable.  *Note Look-Ahead::.

`yyclearin;'
     Discard the current look-ahead token.  This is useful primarily
     in error rules.  *Note Error Recovery::.

`yyerrok;'
     Resume generating error messages immediately for subsequent
     syntax errors.  This is useful primarily in error rules.  *Note
     Error Recovery::.

`@N'
     Acts like a structure variable containing information on the
     line numbers and column numbers of the Nth component of the
     current rule.  The structure has four members, like this:

          struct {
            int first_line, last_line;
            int first_column, last_column;
          };

     Thus, to get the starting line number of the third component,
     use `@3.first_line'.

     In order for the members of this structure to contain valid
     information, you must make `yylex' supply this information about
     each token.  If you need only certain members, then `yylex' need
     only fill in those members.

     The use of this feature makes the parser noticeably slower.



File: bison.info,  Node: Algorithm,  Next: Error Recovery,  Prev: Interface,  Up: Top

The Bison Parser Algorithm
**************************

As Bison reads tokens, it pushes them onto a stack along with their
semantic values.  The stack is called the "parser stack".  Pushing a
token is traditionally called "shifting".

For example, suppose the infix calculator has read `1 + 5 *', with a
`3' to come.  The stack will have four elements, one for each token
that was shifted.

But the stack does not always have an element for each token read. 
When the last N tokens and groupings shifted match the components of
a grammar rule, they can be combined according to that rule.  This is
called "reduction".  Those tokens and groupings are replaced on the
stack by a single grouping whose symbol is the result (left hand
side) of that rule.  Running the rule's action is part of the process
of reduction, because this is what computes the semantic value of the
resulting grouping.

For example, if the infix calculator's parser stack contains this:

     1 + 5 * 3

and the next input token is a newline character, then the last three
elements can be reduced to 15 via the rule:

     expr: expr '*' expr;

Then the stack contains just these three elements:

     1 + 15

At this point, another reduction can be made, resulting in the single
value 16.  Then the newline token can be shifted.

The parser tries, by shifts and reductions, to reduce the entire
input down to a single grouping whose symbol is the grammar's
start-symbol (*note Language and Grammar::.).

This kind of parser is known in the literature as a bottom-up parser.

* Menu:

* Look-Ahead::          Parser looks one token ahead when deciding what to do.
* Shift/Reduce::        Conflicts: when either shifting or reduction is valid.
* Precedence::          Operator precedence works by resolving conflicts.
* Contextual Precedence:: When an operator's precedence depends on context.
* Parser States::       The parser is a finite-state-machine with stack.
* Reduce/Reduce::       When two rules are applicable in the same situation.
* Mystery Conflicts::   Reduce/reduce conflicts that look unjustified.
* Stack Overflow::      What happens when stack gets full.  How to avoid it.

 

File: bison.info,  Node: Look-Ahead,  Next: Shift/Reduce,  Prev: Algorithm,  Up: Algorithm

Look-Ahead Tokens
=================

The Bison parser does *not* always reduce immediately as soon as the
last N tokens and groupings match a rule.  This is because such a
simple strategy is inadequate to handle most languages.  Instead,
when a reduction is possible, the parser sometimes "looks ahead" at
the next token in order to decide what to do.

When a token is read, it is not immediately shifted; first it becomes
the "look-ahead token", which is not on the stack.  Now the parser
can perform one or more reductions of tokens and groupings on the
stack, while the look-ahead token remains off to the side.  When no
more reductions should take place, the look-ahead token is shifted
onto the stack.  This does not mean that all possible reductions have
been done; depending on the token type of the look-ahead token, some
rules may choose to delay their application.

Here is a simple case where look-ahead is needed.  These three rules
define expressions which contain binary addition operators and
postfix unary factorial operators (`!'), and allow parentheses for
grouping.

     expr:     term '+' expr
             | term
             ;
     
     term:     '(' expr ')'
             | term '!'
             | NUMBER
             ;

Suppose that the tokens `1 + 2' have been read and shifted; what
should be done?  If the following token is `)', then the first three
tokens must be reduced to form an `expr'.  This is the only valid
course, because shifting the `)' would produce a sequence of symbols
`term ')'', and no rule allows this.

If the following token is `!', then it must be shifted immediately so
that `2 !' can be reduced to make a `term'.  If instead the parser
were to reduce before shifting, `1 + 2' would become an `expr'.  It
would then be impossible to shift the `!' because doing so would
produce on the stack the sequence of symbols `expr '!''.  No rule
allows that sequence.

The current look-ahead token is stored in the variable `yychar'. 
*Note Action Features::.



File: bison.info,  Node: Shift/Reduce,  Next: Precedence,  Prev: Look-Ahead,  Up: Algorithm

Shift/Reduce Conflicts
======================

Suppose we are parsing a language which has if-then and if-then-else
statements, with a pair of rules like this:

     if_stmt:
               IF expr THEN stmt
             | IF expr THEN stmt ELSE stmt
             ;

(Here we assume that `IF', `THEN' and `ELSE' are terminal symbols for
specific keyword tokens.)

When the `ELSE' token is read and becomes the look-ahead token, the
contents of the stack (assuming the input is valid) are just right
for reduction by the first rule.  But it is also legitimate to shift
the `ELSE', because that would lead to eventual reduction by the
second rule.

This situation, where either a shift or a reduction would be valid,
is called a "shift/reduce conflict".  Bison is designed to resolve
these conflicts by choosing to shift, unless otherwise directed by
operator precedence declarations.  To see the reason for this, let's
contrast it with the other alternative.

Since the parser prefers to shift the `ELSE', the result is to attach
the else-clause to the innermost if-statement, making these two
inputs equivalent:

     if x then if y then win(); else lose;
     
     if x then do; if y then win(); else lose; end;

But if the parser chose to reduce when possible rather than shift,
the result would be to attach the else-clause to the outermost
if-statement, making these two inputs equivalent:

     if x then if y then win(); else lose;
     
     if x then do; if y then win(); end; else lose;

The conflict exists because the grammar as written is ambiguous:
either parsing of the simple nested if-statement is legitimate.  The
established convention is that these ambiguities are resolved by
attaching the else-clause to the innermost if-statement; this is what
Bison accomplishes by choosing to shift rather than reduce.  (It
would ideally be cleaner to write an unambiguous grammar, but that is
very hard to do in this case.) This particular ambiguity was first
encountered in the specifications of Algol 60 and is called the
"dangling `else'" ambiguity.

To avoid warnings from Bison about predictable, legitimate
shift/reduce conflicts, use the `%expect N' declaration.  There will
be no warning as long as the number of shift/reduce conflicts is
exactly N.  *Note Expect Decl::.



File: bison.info,  Node: Precedence,  Next: Contextual Precedence,  Prev: Shift/Reduce,  Up: Algorithm

Operator Precedence
===================

Another situation where shift/reduce conflicts appear is in
arithmetic expressions.  Here shifting is not always the preferred
resolution; the Bison declarations for operator precedence allow you
to specify when to shift and when to reduce.

* Menu:

* Why Precedence::      An example showing why precedence is needed.
* Using Precedence::    How to specify precedence in Bison grammars.
* Precedence Examples:: How these features are used in the previous example.
* How Precedence::      How they work.

 

File: bison.info,  Node: Why Precedence,  Next: Using Precedence,  Prev: Precedence,  Up: Precedence

When Precedence is Needed
-------------------------

Consider the following ambiguous grammar fragment (ambiguous because
the input `1 - 2 * 3' can be parsed in two different ways):

     expr:     expr '-' expr
             | expr '*' expr
             | expr '<' expr
             | '(' expr ')'
             ...
             ;

Suppose the parser has seen the tokens `1', `-' and `2'; should it
reduce them via the rule for the addition operator?  It depends on
the next token.  Of course, if the next token is `)', we must reduce;
shifting is invalid because no single rule can reduce the token
sequence `- 2 )' or anything starting with that.  But if the next
token is `*' or `<', we have a choice: either shifting or reduction
would allow the parse to complete, but with different results.

To decide which one Bison should do, we must consider the results. 
If the next operator token OP is shifted, then it must be reduced
first in order to permit another opportunity to reduce the sum.  The
result is (in effect) `1 - (2 OP 3)'.  On the other hand, if the
subtraction is reduced before shifting OP, the result is
`(1 - 2) OP 3'.  Clearly, then, the choice of shift or reduce
should depend on the relative precedence of the operators `-' and OP:
`*' should be shifted first, but not `<'.

What about input such as `1 - 2 - 5'; should this be `(1 - 2) - 5' or
should it be `1 - (2 - 5)'?  For most operators we prefer the former,
which is called "left association".  The latter alternative, "right
association", is desirable for assignment operators.  The choice of
left or right association is a matter of whether the parser chooses
to shift or reduce when the stack contains `1 - 2' and the look-ahead
token is `-': shifting makes right-associativity.



File: bison.info,  Node: Using Precedence,  Next: Precedence Examples,  Prev: Why Precedence,  Up: Precedence

Specifying Operator Precedence
------------------------------

Bison allows you to specify these choices with the operator
precedence declarations `%left' and `%right'.  Each such declaration
contains a list of tokens, which are operators whose precedence and
associativity is being declared.  The `%left' declaration makes all
those operators left-associative and the `%right' declaration makes
them right-associative.  A third alternative is `%nonassoc', which
declares that it is a syntax error to find the same operator twice
"in a row".

The relative precedence of different operators is controlled by the
order in which they are declared.  The first `%left' or `%right'
declaration in the file declares the operators whose precedence is
lowest, the next such declaration declares the operators whose
precedence is a little higher, and so on.



File: bison.info,  Node: Precedence Examples,  Next: How Precedence,  Prev: Using Precedence,  Up: Precedence

Precedence Examples
-------------------

In our example, we would want the following declarations:

     %left '<'
     %left '-'
     %left '*'

In a more complete example, which supports other operators as well,
we would declare them in groups of equal precedence.  For example,
`'+'' is declared with `'-'':

     %left '<' '>' '=' NE LE GE
     %left '+' '-'
     %left '*' '/'

(Here `NE' and so on stand for the operators for "not equal" and so
on.  We assume that these tokens are more than one character long and
therefore are represented by names, not character literals.)



File: bison.info,  Node: How Precedence,  Prev: Precedence Examples,  Up: Precedence

How Precedence Works
--------------------

The first effect of the precedence declarations is to assign
precedence levels to the terminal symbols declared.  The second
effect is to assign precedence levels to certain rules: each rule
gets its precedence from the last terminal symbol mentioned in the
components.  (You can also specify explicitly the precedence of a
rule.  *Note Contextual Precedence::.)

Finally, the resolution of conflicts works by comparing the
precedence of the rule being considered with that of the look-ahead
token.  If the token's precedence is higher, the choice is to shift. 
If the rule's precedence is higher, the choice is to reduce.  If they
have equal precedence, the choice is made based on the associativity
of that precedence level.  The verbose output file made by `-v'
(*note Invocation::.) says how each conflict was resolved.

Not all rules and not all tokens have precedence.  If either the rule
or the look-ahead token has no precedence, then the default is to
shift.



File: bison.info,  Node: Contextual Precedence,  Next: Parser States,  Prev: Precedence,  Up: Algorithm

Context-Dependent Precedence
============================

Often the precedence of an operator depends on the context.  This
sounds outlandish at first, but it is really very common.  For
example, a minus sign typically has a very high precedence as a unary
operator, and a somewhat lower precedence (lower than multiplication)
as a binary operator.

The Bison precedence declarations, `%left', `%right' and `%nonassoc',
can only be used once for a given token; so a token has only one
precedence declared in this way.  For context-dependent precedence,
you need to use an additional mechanism: the `%prec' modifier for
rules.

The `%prec' modifier declares the precedence of a particular rule by
specifying a terminal symbol whose predecence should be used for that
rule.  It's not necessary for that symbol to appear otherwise in the
rule.  The modifier's syntax is:

     %prec TERMINAL-SYMBOL

and it is written after the components of the rule.  Its effect is to
assign the rule the precedence of TERMINAL-SYMBOL, overriding the
precedence that would be deduced for it in the ordinary way.  The
altered rule precedence then affects how conflicts involving that
rule are resolved (*note Precedence::.).

Here is how `%prec' solves the problem of unary minus.  First,
declare a precedence for a fictitious terminal symbol named `UMINUS'.
There are no tokens of this type, but the symbol serves to stand for
its precedence:

     ...
     %left '+' '-'
     %left '*'
     %left UMINUS

Now the precedence of `UMINUS' can be used in specific rules:

     exp:    ...
             | exp '-' exp
             ...
             | '-' exp %prec UMINUS



File: bison.info,  Node: Parser States,  Next: Reduce/Reduce,  Prev: Contextual Precedence,  Up: Algorithm

Parser States
=============

The function `yyparse' is implemented using a finite-state machine. 
The values pushed on the parser stack are not simply token type
codes; they represent the entire sequence of terminal and nonterminal
symbols at or near the top of the stack.  The current state collects
all the information about previous input which is relevant to
deciding what to do next.

Each time a look-ahead token is read, the current parser state
together with the type of look-ahead token are looked up in a table. 
This table entry can say, "Shift the look-ahead token."  In this
case, it also specifies the new parser state, which is pushed onto
the top of the parser stack.  Or it can say, "Reduce using rule
number N." This means that a certain of tokens or groupings are taken
off the top of the stack, and replaced by one grouping.  In other
words, that number of states are popped from the stack, and one new
state is pushed.

There is one other alternative: the table can say that the look-ahead
token is erroneous in the current state.  This causes error
processing to begin (*note Error Recovery::.).



File: bison.info,  Node: Reduce/Reduce,  Next: Mystery Conflicts,  Prev: Parser States,  Up: Algorithm

Reduce/Reduce Conflicts
=======================

A reduce/reduce conflict occurs if there are two or more rules that
apply to the same sequence of input.  This usually indicates a
serious error in the grammar.

For example, here is an erroneous attempt to define a sequence of
zero or more `word' groupings.

     sequence: /* empty */
                     { printf ("empty sequence\n"); }
             | word
                     { printf ("single word %s\n", $1); }
             | sequence word
                     { printf ("added word %s\n", $2); }
             ;

The error is an ambiguity: there is more than one way to parse a
single `word' into a `sequence'.  It could be reduced directly via
the second rule.  Alternatively, nothing-at-all could be reduced into
a `sequence' via the first rule, and this could be combined with the
`word' using the third rule.

You might think that this is a distinction without a difference,
because it does not change whether any particular input is valid or
not.  But it does affect which actions are run.  One parsing order
runs the second rule's action; the other runs the first rule's action
and the third rule's action.  In this example, the output of the
program changes.

Bison resolves a reduce/reduce conflict by choosing to use the rule
that appears first in the grammar, but it is very risky to rely on
this.  Every reduce/reduce conflict must be studied and usually
eliminated.  Here is the proper way to define `sequence':

     sequence: /* empty */
                     { printf ("empty sequence\n"); }
             | sequence word
                     { printf ("added word %s\n", $2); }
             ;

Here is another common error that yields a reduce/reduce conflict:

     sequence: /* empty */
             | sequence words
             | sequence redirects
             ;
     
     words:    /* empty */
             | words word
             ;
     
     redirects:/* empty */
             | redirects redirect
             ;

The intention here is to define a sequence which can contain either
`word' or `redirect' groupings.  The individual definitions of
`sequence', `words' and `redirects' are error-free, but the three
together make a subtle ambiguity: even an empty input can be parsed
in infinitely many ways!

Consider: nothing-at-all could be a `words'.  Or it could be two
`words' in a row, or three, or any number.  It could equally well be
a `redirects', or two, or any number.  Or it could be a `words'
followed by three `redirects' and another `words'.  And so on.

Here are two ways to correct these rules.  First, to make it a single
level of sequence:

     sequence: /* empty */
             | sequence word
             | sequence redirect
             ;

Second, to prevent either a `words' or a `redirects' from being empty:

     sequence: /* empty */
             | sequence words
             | sequence redirects
             ;
     
     words:    word
             | words word
             ;
     
     redirects:redirect
             | redirects redirect
             ;



File: bison.info,  Node: Mystery Conflicts,  Next: Stack Overflow,  Prev: Reduce/Reduce,  Up: Algorithm

Mysterious Reduce/Reduce Conflicts
==================================

Sometimes reduce/reduce conflicts can occur that don't look warranted.
Here is an example:

     %token ID
     
     %%
     def:    param_spec return_spec ','
             ;
     param_spec:
                  type
             |    name_list ':' type
             ;
     return_spec:
                  type
             |    name ':' type
             ;
     type:        ID
             ;
     name:        ID
             ;
     name_list:
                  name
             |    name ',' name_list
             ;

It would seem that this grammar can be parsed with only a single
token of look-ahead: when a `param_spec' is being read, an `ID' is  a
`name' if a comma or colon follows, or a `type' if another `ID'
follows.  In other words, this grammar is LR(1).

However, Bison, like most parser generators, cannot actually handle
all LR(1) grammars.  In this grammar, two contexts, that after an
`ID' at the beginning of a `param_spec' and likewise at the beginning
of a `return_spec', are similar enough that Bison assumes they are
the same.  They appear similar because the same set of rules would be
active--the rule for reducing to a `name' and that for reducing to a
`type'.  Bison is unable to determine at that stage of processing
that the rules would require different look-ahead tokens in the two
contexts, so it makes a single parser state for them both.  Combining
the two contexts causes a conflict later.  In parser terminology,
this occurrence means that the grammar is not LALR(1).

In general, it is better to fix deficiencies than to document them. 
But this particular deficiency is intrinsically hard to fix; parser
generators that can handle LR(1) grammars are hard to write and tend
to produce parsers that are very large.  In practice, Bison is more
useful as it is now.

When the problem arises, you can often fix it by identifying the two
parser states that are being confused, and adding something to make
them look distinct.  In the above example, adding one rule to
`return_spec' as follows makes the problem go away:

     %token BOGUS
     ...
     %%
     ...
     return_spec:
                  type
             |    name ':' type
             /* This rule is never used.  */
             |    ID BOGUS
             ;

This corrects the problem because it introduces the possibility of an
additional active rule in the context after the `ID' at the beginning
of `return_spec'.  This rule is not active in the corresponding
context in a `param_spec', so the two contexts receive distinct
parser states.  As long as the token `BOGUS' is never generated by
`yylex', the added rule cannot alter the way actual input is parsed.

In this particular example, there is another way to solve the
problem: rewrite the rule for `return_spec' to use `ID' directly
instead of via `name'.  This also causes the two confusing contexts
to have different sets of active rules, because the one for
`return_spec' activates the altered rule for `return_spec' rather
than the one for `name'.

     param_spec:
                  type
             |    name_list ':' type
             ;
     return_spec:
                  type
             |    ID ':' type
             ;



File: bison.info,  Node: Stack Overflow,  Prev: Mystery Conflicts,  Up: Algorithm

Stack Overflow, and How to Avoid It
===================================

The Bison parser stack can overflow if too many tokens are shifted
and not reduced.  When this happens, the parser function `yyparse'
returns a nonzero value, pausing only to call `yyerror' to report the
overflow.

By defining the macro `YYMAXDEPTH', you can control how deep the
parser stack can become before a stack overflow occurs.  Define the
macro with a value that is an integer.  This value is the maximum
number of tokens that can be shifted (and not reduced) before overflow.
It must be a constant expression whose value is known at compile time.

The stack space allowed is not necessarily allocated.  If you specify
a large value for `YYMAXDEPTH', the parser actually allocates a small
stack at first, and then makes it bigger by stages as needed.  This
increasing allocation happens automatically and silently.  Therefore,
you do not need to make `YYMAXDEPTH' painfully small merely to save
space for ordinary inputs that do not need much stack.

The default value of `YYMAXDEPTH', if you do not define it, is 10000.

You can control how much stack is allocated initially by defining the
macro `YYINITDEPTH'.  This value too must be a compile-time constant
integer.  The default is 200.



File: bison.info,  Node: Error Recovery,  Next: Context Dependency,  Prev: Algorithm,  Up: Top

Error Recovery
**************

It is not usually acceptable to have a program terminate on a parse
error.  For example, a compiler should recover sufficiently to parse
the rest of the input file and check it for errors; a calculator
should accept another expression.

In a simple interactive command parser where each input is one line,
it may be sufficient to allow `yyparse' to return 1 on error and have
the caller ignore the rest of the input line when that happens (and
then call `yyparse' again).  But this is inadequate for a compiler,
because it forgets all the syntactic context leading up to the error.
A syntax error deep within a function in the compiler input should
not cause the compiler to treat the following line like the beginning
of a source file.

You can define how to recover from a syntax error by writing rules to
recognize the special token `error'.  This is a terminal symbol that
is always defined (you need not declare it) and reserved for error
handling.  The Bison parser generates an `error' token whenever a
syntax error happens; if you have provided a rule to recognize this
token in the current context, the parse can continue.  For example:

     stmnts:  /* empty string */
             | stmnts '\n'
             | stmnts exp '\n'
             | stmnts error '\n'

The fourth rule in this example says that an error followed by a
newline makes a valid addition to any `stmnts'.

What happens if a syntax error occurs in the middle of an `exp'?  The
error recovery rule, interpreted strictly, applies to the precise
sequence of a `stmnts', an `error' and a newline.  If an error occurs
in the middle of an `exp', there will probably be some additional
tokens and subexpressions on the stack after the last `stmnts', and
there will be tokens to read before the next newline.  So the rule is
not applicable in the ordinary way.

But Bison can force the situation to fit the rule, by discarding part
of the semantic context and part of the input.  First it discards
states and objects from the stack until it gets back to a state in
which the `error' token is acceptable.  (This means that the
subexpressions already parsed are discarded, back to the last
complete `stmnts'.)  At this point the `error' token can be shifted. 
Then, if the old look-ahead token is not acceptable to be shifted
next, the parser reads tokens and discards them until it finds a
token which is acceptable.  In this example, Bison reads and discards
input until the next newline so that the fourth rule can apply.

The choice of error rules in the grammar is a choice of strategies
for error recovery.  A simple and useful strategy is simply to skip
the rest of the current input line or current statement if an error
is detected:

     stmnt: error ';'  /* on error, skip until ';' is read */

It is also useful to recover to the matching close-delimiter of an
opening-delimiter that has already been parsed.  Otherwise the
close-delimiter will probably appear to be unmatched, and generate
another, spurious error message:

     primary:  '(' expr ')'
             | '(' error ')'
             ...
             ;

Error recovery strategies are necessarily guesses.  When they guess
wrong, one syntax error often leads to another.  In the above
example, the error recovery rule guesses that an error is due to bad
input within one `stmnt'.  Suppose that instead a spurious semicolon
is inserted in the middle of a valid `stmnt'.  After the error
recovery rule recovers from the first error, another syntax error
will be found straightaway, since the text following the spurious
semicolon is also an invalid `stmnt'.

To prevent an outpouring of error messages, the parser will output no
error message for another syntax error that happens shortly after the
first; only after three consecutive input tokens have been
successfully shifted will error messages resume.

Note that rules which accept the `error' token may have actions, just
as any other rules can.

You can make error messages resume immediately by using the macro
`yyerrok' in an action.  If you do this in the error rule's action,
no error messages will be suppressed.  This macro requires no
arguments; `yyerrok;' is a valid C statement.

The previous look-ahead token is reanalyzed immediately after an
error.  If this is unacceptable, then the macro `yyclearin' may be
used to clear this token.  Write the statement `yyclearin;' in the
error rule's action.

For example, suppose that on a parse error, an error handling routine
is called that advances the input stream to some point where parsing
should once again commence.  The next symbol returned by the lexical
scanner is probably correct.  The previous look-ahead token ought to
be discarded with `yyclearin;'.

The macro `YYRECOVERING' stands for an expression that has the value
1 when the parser is recovering from a syntax error, and 0 the rest
of the time.  A value of 1 indicates that error messages are
currently suppressed for new syntax errors.



File: bison.info,  Node: Context Dependency,  Next: Debugging,  Prev: Error Recovery,  Up: Top

Handling Context Dependencies
*****************************

The Bison paradigm is to parse tokens first, then group them into
larger syntactic units.  In many languages, the meaning of a token is
affected by its context.  Although this violates the Bison paradigm,
certain techniques (known as "kludges") may enable you to write Bison
parsers for such languages.

* Menu:

* Semantic Tokens::     Token parsing can depend on the semantic context.
* Lexical Tie-ins::     Token parsing can depend on the syntactic context.
* Tie-in Recovery::     Lexical tie-ins have implications for how
                          error recovery rules must be written.

 (Actually, "kludge" means any technique that gets its job done but is
neither clean nor robust.)



File: bison.info,  Node: Semantic Tokens,  Next: Lexical Tie-ins,  Prev: Context Dependency,  Up: Context Dependency

Semantic Info in Token Types
============================

The C language has a context dependency: the way an identifier is
used depends on what its current meaning is.  For example, consider
this:

     foo (x);

This looks like a function call statement, but if `foo' is a typedef
name, then this is actually a declaration of `x'.  How can a Bison
parser for C decide how to parse this input?

The method used in GNU C is to have two different token types,
`IDENTIFIER' and `TYPENAME'.  When `yylex' finds an identifier, it
looks up the current declaration of the identifier in order to decide
which token type to return: `TYPENAME' if the identifier is declared
as a typedef, `IDENTIFIER' otherwise.

The grammar rules can then express the context dependency by the
choice of token type to recognize.  `IDENTIFIER' is accepted as an
expression, but `TYPENAME' is not.  `TYPENAME' can start a
declaration, but `IDENTIFIER' cannot.  In contexts where the meaning
of the identifier is *not* significant, such as in declarations that
can shadow a typedef name, either `TYPENAME' or `IDENTIFIER' is
accepted--there is one rule for each of the two token types.

This technique is simple to use if the decision of which kinds of
identifiers to allow is made at a place close to where the identifier
is parsed.  But in C this is not always so: C allows a declaration to
redeclare a typedef name provided an explicit type has been specified
earlier:

     typedef int foo, bar, lose;
     static foo (bar);        /* redeclare `bar' as static variable */
     static int foo (lose);   /* redeclare `foo' as function */

Unfortunately, the name being declared is separated from the
declaration construct itself by a complicated syntactic
structure--the "declarator".

As a result, the part of Bison parser for C needs to be duplicated,
with all the nonterminal names changed: once for parsing a
declaration in which a typedef name can be redefined, and once for
parsing a declaration in which that can't be done.  Here is a part of
the duplication, with actions omitted for brevity:

     initdcl:
               declarator maybeasm '='
               init
             | declarator maybeasm
             ;
     
     notype_initdcl:
               notype_declarator maybeasm '='
               init
             | notype_declarator maybeasm
             ;

Here `initdcl' can redeclare a typedef name, but `notype_initdcl'
cannot.  The distinction between `declarator' and `notype_declarator'
is the same sort of thing.

There is some similarity between this technique and a lexical tie-in
(described next), in that information which alters the lexical
analysis is changed during parsing by other parts of the program. 
The difference is here the information is global, and is used for
other purposes in the program.  A true lexical tie-in has a
special-purpose flag controlled by the syntactic context.



File: bison.info,  Node: Lexical Tie-ins,  Next: Tie-in Recovery,  Prev: Semantic Tokens,  Up: Context Dependency

Lexical Tie-ins
===============

One way to handle context-dependency is the "lexical tie-in": a flag
which is set by Bison actions, whose purpose is to alter the way
tokens are parsed.

For example, suppose we have a language vaguely like C, but with a
special construct `hex (HEX-EXPR)'.  After the keyword `hex' comes an
expression in parentheses in which all integers are hexadecimal.  In
particular, the token `a1b' must be treated as an integer rather than
as an identifier if it appears in that context.  Here is how you can
do it:

     %{
     int hexflag;
     %}
     %%
     ...
     expr:   IDENTIFIER
             | constant
             | HEX '('
                     { hexflag = 1; }
               expr ')'
                     { hexflag = 0;
                        $$ = $4; }
             | expr '+' expr
                     { $$ = make_sum ($1, $3); }
             ...
             ;
     
     constant:
               INTEGER
             | STRING
             ;

Here we assume that `yylex' looks at the value of `hexflag'; when it
is nonzero, all integers are parsed in hexadecimal, and tokens
starting with letters are parsed as integers if possible.

The declaration of `hexflag' shown in the C declarations section of
the parser file is needed to make it accessible to the actions (*note
C Declarations::.).  You must also write the code in `yylex' to obey
the flag.



File: bison.info,  Node: Tie-in Recovery,  Prev: Lexical Tie-ins,  Up: Context Dependency

Lexical Tie-ins and Error Recovery
==================================

Lexical tie-ins make strict demands on any error recovery rules you
have.  *Note Error Recovery::.

The reason for this is that the purpose of an error recovery rule is
to abort the parsing of one construct and resume in some larger
construct.  For example, in C-like languages, a typical error
recovery rule is to skip tokens until the next semicolon, and then
start a new statement, like this:

     stmt:   expr ';'
             | IF '(' expr ')' stmt { ... }
             ...
             error ';'
                     { hexflag = 0; }
             ;

If there is a syntax error in the middle of a `hex (EXPR)' construct,
this error rule will apply, and then the action for the completed
`hex (EXPR)' will never run.  So `hexflag' would remain set for the
entire rest of the input, or until the next `hex' keyword, causing
identifiers to be misinterpreted as integers.

To avoid this problem the error recovery rule itself clears `hexflag'.

There may also be an error recovery rule that works within expressions.
For example, there could be a rule which applies within parentheses
and skips to the close-parenthesis:

     expr:   ...
             | '(' expr ')'
                     { $$ = $2; }
             | '(' error ')'
             ...

 If this rule acts within the `hex' construct, it is not going to
abort that construct (since it applies to an inner level of
parentheses within the construct).  Therefore, it should not clear
the flag: the rest of the `hex' construct should be parsed with the
flag still in effect.

What if there is an error recovery rule which might abort out of the
`hex' construct or might not, depending on circumstances?  There is
no way you can write the action to determine whether a `hex'
construct is being aborted or not.  So if you are using a lexical
tie-in, you had better make sure your error recovery rules are not of
this kind.  Each rule must be such that you can be sure that it
always will, or always won't, have to clear the flag.


